Giant tubular fullerenes are prototypes of finite-length end-capped carbon nanotubes (CNTs) which can be accurately characterized by single-crystal X-ray diffraction. We present here the synthesis, isolation and characterization of several novel tubular fullerene cages stabilized by the encapsulation of a large and streched La2C2 cluster, namely, La2C2@C92-C104. The La2C2 cluster no longer has butterfly configuration in large cages such as (450)-C100, Cs(574)-C102 and D3d(822)-C104 , butit adopts a twisted and nearly planar geometry, which is obviously dependent on the cage symmetry. The strong metal-cage interactions are proved to be responsible for the stabilization of these giant fullerene cages. Furthermore, the analysis of the transformation processes from  large cages to the smaller ones provides crystallographic evidences for the formation mechanism of ideal CNTs from defective ones at the atomic level as probed with single-crystal crystallography, which is instructive to future works of the selective synthesis of ideal CNTs with identical diameter and chirality.